Catalytic converters containing various catalysts have been employed for years by automobile manufacturers to meet the ever-more-stringent regulations on emissions of hydrocarbon, carbon monoxide, and particularly, nitrogen oxides from internal combustion engines. Concurrently with the promulgation of these ever-tightening regulations on emissions, automotive engine manufacturers have worked to improve the fuel economy of such engines. This effort has led to the development of engines that operate with an excess of air beyond that which is required to consume stoichiometrically the fuel admitted to such engines ("lean-burn engines"). The advent of lean-burn engines has, in turn, exacerbated the problem of reducing emission of nitrogen oxides from internal combustion engines.
Various techniques have been developed to treat exhaust gas from internal combustion engines. One approach has been to provide multiple catalyst chambers in the exhaust system filled with different catalysts suited to particular tasks, such as that taught by Takeshima, et al. in U.S. Pat. No. 5,233,830.
Another approach is to provide a control system operating in conjunction with a catalytic converter, such as taught by Hirota, et al. in U.S. Pat. No. 5,201,802 and U.S. Pat. No. 5,189,876, and also by Takeshima in U.S. Pat. No. 5,029,061.
Clearly, there is a need for a simple system capable of treating exhaust gas from internal combustion engines, particularly the exhaust gas from lean-burn engines. The present invention is such a system, employing as it does a single catalytic converter having a single, unique catalyst contained therein which is capable of reducing the amount of hydrocarbon, carbon monoxide and, particularly, nitrogen oxides contained in exhaust gas prior to such gas's discharge to the atmosphere.
As stated above, the key element in such a system is the discovery of a catalyst which is effective in oxidizing hydrocarbon and carbon monoxide while being surprisingly effective at concurrently reducing nitrogen oxides in the environment presented by exhaust gas from internal combustion engines. Numerous inventions in this area have been taught by others skilled in the art, but none of these resembles more than superficially the catalyst of the present invention. For instance, U.S. Pat. No. 5,376,610 (Takahata, et al.) teaches, as part of a multi-catalyst system for exhaust gas purification, a catalyst consisting of alumina, and optionally zeolite, with Pt and/or Pd impregnated therein.
U.S. Pat. No. 5,292,991 (Lachman, et al.) teaches a catalyst consisting of a mixture of alumina and zeolite on a substrate with noble metal predominately dispersed on the alumina.
Patents have also been granted for catalysts intended for other uses, and some of these more closely resemble, at least superficially, the catalyst of the present invention. U.S. Pat. No. 5,314,854 (Galperin) teaches a catalyst designed primarily for the dehydrocyclization of paraffins to aromatics that consists of large pore molecular sieves mixed with binder and formed into pellets, spheres, or other such particles, said particles then having a surface-layer palladium component and uniformly dispersed platinum.
U.S. Pat. No. 4,992,158 (Schweizer) teaches a catalyst for reforming hydrocarbons having boiling points in the range of motor gasoline that is characterized as L-type zeolite not fully impregnated with Pt or Pd, using Pt- or Pd- acetylacetonate as the noble metal compound to treat the L-zeolite.
U.S. Pat. No. 5,017,541 (Schmidt, et al.) teaches a catalyst for the isomerization of alkanes consisting of particles of refractory inorganic oxide having a slightly higher concentration of a metal from the platinum group at the surface of such particles than at their center. The width of the surface-enriched band is 100-150 .mu.m while the particles themselves have diameters of about 0.7-3.3 .mu.m.
Still more patents have been granted for the preparation of noble metal/zeolite catalysts without regard to the use of same. U.S. Pat. No. 4,556,646 (Bezman) teaches a method for the preparation of a catalyst consisting of an inorganic oxide, a zeolite, and a noble metal in which ammonium nitrate is employed as an impregnating agent for Pd to produce a uniform distribution of the metal throughout the catalyst particles. Data are presented showing that, if ammonium nitrate is not used, Pd is concentrated in the outer shell of the catalyst particles. In this case, however, the Pd is deposited on the inorganic oxide as well as the zeolite, and the inventor suggests that deposition of Pd in the outer shell is a condition to be avoided.
U.S. Pat. No. 4,882,307 (Tsao) teaches the use of various salts, including sulfites, of cationic Pt complexes as an agent for the deposition of Pt into zeolite as a way of yielding highly dispersed Pt crystallites, after appropriate treatments.
U.S. Pat. No. 4,683,214 (Angevine, et al.) also teaches the use of sulfite salts of cationic noble metal complexes as a means of depositing noble metal on zeolite to yield, after appropriate treatment, very highly dispersed metal crystallites.
And, finally, U.S. Pat. No. 5,290,534 (Tsao) teaches that when a noble metal precursor is placed into a reactor during the production or synthesis of ZSM-18 substantially all of the noble metal remains on the outside of the synthesized ZSM-18. No indication is given that this is a desirable result, however, since the point of this invention is to provide a method of removing organic material blocking the pores of a zeolite.